The invention relates to float glass production and, more particularly, to a method and apparatus for reducing the formation of tin oxide in a float bath.
In the float glass process, a glass ribbon floats on a surface of molten metal contained within a float bath chamber. The interior volume of the float bath chamber is generally divided between the bath of molten metal, typically molten tin, and a gaseous atmosphere above the molten metal to prevent oxidation. Nonetheless, oxidation of the molten tin still occurs by either small amounts of oxygen penetrating the float bath chamber and/or oxygen being released into the molten tin from the glass ribbon.
Oxygen is released into the molten tin from the glass ribbon in several ways. Oxygen can be released from the top surface of the glass ribbon and oxidize the molten tin through gaseous diffusion. Additionally, oxygen can be directly released into the molten tin through the contact between the molten tin and the bottom surface of the glass ribbon. In the float bath chamber, oxygen will react with the molten tin to create tin oxide. The majority of the tin oxide created will be in the form of SnO but some amount of SnO2 may also exist.
In the float bath chamber, tin oxide exists as a solid. A portion of the tin oxide will float on the surface of the molten tin and the remaining tin oxide will dissolve into the molten tin. The amount of tin oxide floating on and the amount of tin oxide dissolved in the molten tin will be determined based on the total amount of tin oxide present and the temperature of the molten tin. For instance, the higher the temperature in the float bath chamber, the greater the solubility of tin oxide will be in the molten tin.
Tin oxide in the float bath chamber can cause defects in the glass ribbon. For example, as the bottom surface of the glass ribbon contacts the molten tin some tin oxide, in the form of SnO, will be absorbed into the glass. Later, when the glass is further processed, such as during lehring, tempering, or the toughening process, this SnO will further oxidize to SnO2 resulting in the glass article having a higher haze value. Therefore, removal of oxygen and elimination of tin oxide in the float chamber is highly desirable.
It is known to use a reducing gas, such as hydrogen, to scavenge oxygen from a float bath. In order to minimize the amount of tin oxide undesirably formed by reaction of oxygen with the molten tin, large quantities of reducing gases are typically used to maintain a thermodynamic chemical equilibrium in the float bath. However, using large quantities of reducing gases can also produce undesirable effects. For instance, hydrogen in the bath atmosphere can react with gaseous tin sulfide as well as condensed tin sulfide producing liquid tin droplets on surfaces in the float bath chamber which can “drip down” onto the glass ribbon causing additional glass defects. Thus, those skilled in the art continue to seek a way to reduce the formation of the tin oxide in the float bath in a cost effective manner.